1. Field of the Invention
The present invention relates to an optical fiber preferably used for high-speed optical communication, optical module and Raman amplifier using the optical fiber.
2. Related Background Art
A dispersion compensating technique is indispensable for realizing high-speed optical communication, and a dispersion compensating fiber (DCF) is generally used in the technique. The DCF is an optical fiber for compensating dispersion of an optical fiber in a used wavelength band, for example in a set wavelength band in a wavelength range of 1460 to 1625 nm. To provide a characteristic suitable for dispersion compensation, it is necessary to increase a relative refractive index difference of the center core of the DCF which is located in the center of the DCF, and therefore, the center core is normally doped with germanium of high concentration.
An optical signal processing technique which processes an optical signal as light itself without converting it to an electrical signal is required for high-speed optical communication systems. This technique requires an optical fiber that can produce a large nonlinear phenomenon, that is, a high nonlinear optical fiber. This high nonlinear optical fiber also needs a large relative refractive index difference of its center core, and therefore, the center core is doped with germanium of high concentration.
However, when the center core is doped with germanium of high concentration, the nonlinear refractive index increases, which results in a problem of provoking waveform distortion. The high nonlinear optical fiber has a characteristic in which waveform distortion occurs when the nonlinear coefficient is increased. But, since the high nonlinear optical fiber for a Raman amplifier has the problem of double Rayleigh scattering, the germanium concentration of the center core is now being questioned.
Furthermore, when the DCF or the high nonlinear optical fiber is drawn, many defects and structural inconsistency are caused in the center core during drawing because the glass softening point of the center core is considerably different from that of the cladding. Due to this fact, there is a problem that it is difficult to reduce the transmission loss of the optical fiber.